A variety of techniques are available for providing visual displays of graphical or video images to a user. In many applications cathode ray tube type displays (CRTs), such as televisions and computer monitors, produce images for viewing. Such devices suffer from several limitations. For example, typical CRTs are bulky and consume substantial amounts of power, making them undesirable for portable or head-mounted applications.
Matrix addressable displays, such as liquid crystal displays and field emission displays, may be less bulky and consume less power. However, typical matrix addressable displays utilize screens that are several inches across. Such screens have limited use in head mounted applications or in applications where the display is intended to occupy only a small portion of a user's field-of-view. Such displays have been reduced in size at the cost of increasingly difficult processing and limited resolution or brightness. Also, improving resolution of such displays typically requires a significant increase in complexity.
Another form of display is a scanned beam display. Various scanned beam displays have been disclosed in the art, such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. In one example of a scanned beam display, a scanning source outputs a beam of coherent light that is reflected by a mirror in a MEMS scanner onto a viewer's retina. The scanned light enters the viewer's eye through the viewer's pupil and is directed onto the retina by the cornea and lens. The intensity of the light from the light source is modulated as the beam is scanned horizontally and vertically so that the viewer perceives an image. In other examples, the scanning source may include one or more modulated light emitters that are rotated through an angular sweep to scan the light onto the viewer's retina. One example of such a scanner suitable for either type of display is described in U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference.
Scanned beam displays are sometimes used for partial or augmented view applications in which a portion of the display is positioned in the user's field-of-view to create an image that occupies a region of the user's field-of-view. The user can thus see both a displayed virtual image and a background image. If the background light is occluded, the viewer perceives only the virtual image. Applications for see-through and occluded scanned beam displays include head-mounted displays and camera electronic viewfinders, for example.
A common requirement of scanned beam displays has been the need to collimate the light into a beam having a relatively low numerical aperture, i.e., a small divergence angle, prior to scanning the beam across the field-of-view. Unfortunately, providing a collimated, low numerical aperture beam of light frequently employs relatively expensive coherent light sources such as lasers, or edge-emitting light emitting diodes (“EELED”). Such collimated light sources may result in low optical efficiency and have the effect of producing a dimly lit display.
Scanned beam displays frequently require a complex set of optics to generate a scanned beam image. In one example, light from a low numerical aperture light source is collected by a set of optics to form a beam. This light beam is then directed to a bi-axial scanner, which scans the light beam into a set of optics that images the light beam onto an exit pupil expander (“EPE”) placed at an intermediate image plane. The expanded image is then conveyed by another set of optics that translates positional information from the EPE into angular information at the viewer's pupil. In such displays, the scanner outputs angular information that is converted to positional information at the EPE, only to have this positional information converted back to angular information through the viewing optics. The large number of optical components used by these types of complex imaging systems results in displays that are relatively bulky and expensive.